As the description says, Celestia is like Google Earth for the Universe. With a few of the add-ons, it's brilliant. For example, you can set it up to randomly travel the local area of our galaxy, similar to the common starfield screensaver we all know and love, but here's the twist. Stop it, and click on any star, and you can get the name of that star, travel there, see if there are any planets around it and more. With a database available of over 2 millions stars, it's very comprehensive and educational. Well worth the download.